In the chemical, biomedical, bioscience and pharmaceutical industries, it has become increasingly desirable to perform large numbers of chemical operations, such as reactions, separations and subsequent detection steps, in a highly parallel fashion. The high throughput synthesis, screening and analysis of (bio)chemical compounds, enables the economic discovery of new drugs and drug candidates, and the implementation of sophisticated medical diagnostic equipment. Of key importance for the improvement of the chemical operations required in these applications are an increased speed, enhanced reproducibility, decreased consumption of expensive samples and reagents, and the reduction of waste materials.
Microfluidic devices and systems provide improved methods of performing chemical, biochemical and biological analysis and synthesis. Microfluidic devices and systems allow for the performance of multi-step, multi-species chemical operations in chip-based micro chemical analysis systems. Chip-based microfluidic systems generally comprise conventional ‘microfluidic’ elements, particularly capable of handling and analyzing chemical and biological specimens. Typically, the term microfluidic in the art refers to systems or devices having a network of processing nodes, chambers and reservoirs connected by channels, in which the channels have typical cross-sectional dimensions in the range between about 1.0 μm and about 500 μm. In the art, channels having these cross-sectional dimensions are referred to as ‘microchannels’.
By performing the chemical operations in a microfluidic system, potentially a number of the above-mentioned desirable improvements can be realized. Downscaling dimensions allows for diffusional processes, such as heating, cooling and passive transport of species (diffusional mass-transport), to proceed faster. One example is the thermal processing of liquids, which is typically a required step in chemical synthesis and analysis. In comparison with the heating and cooling of liquids in beakers as performed in a conventional laboratory setting, the thermal processing of liquids is accelerated in a microchannel due to reduced diffusional distances. Another example of the efficiency of microfluidic systems is the mixing of dissolved species in a liquid, a process that is also diffusion limited. Downscaling the typical dimensions of the mixing chamber thereby reduces the typical distance to be overcome by diffusional mass-transport, and consequently results in a reduction of mixing times. Like thermal processing, the mixing of dissolved chemical species, such as reagents, with a sample or precursors for a synthesis step, is an operation that is required in virtually all chemical synthesis and analysis processes. Therefore, the ability to reduce the time involved in mixing provides significant advantages to most chemical synthesis and analysis processes.
Furthermore, reduced dimensions enhance separation operations utilized in chemical synthesis and analysis processes. One example is capillary electrophoresis, which is a separation technology based on the migration of dissolved charged species through a liquid filled capillary by the application of a longitudinal electric field. By reducing the cross-sectional size of the capillaries, the separation efficiency can greatly be improved, thereby resulting in rapid separations. For examples, see Effenhauser et al., Anal. Chem. 65:2637–2642 October (1993), Effenhauser et al., Anal. Chem. 66:2949–2953 September (1994), Jacobson et al., Anal. Chem. 66:4127–4132 December (1994) and Jacobson et al., Anal. Chem. 66:1114–1118 April (1994).
Another aspect of the reduction of dimensions is the reduction of required volumes of sample, reagents, precursors and other often very expensive chemical substances. Milliliter-sized systems typically require milliliter volumes of these substances, while microliter sized microfluidic systems only require microliters volumes. The ability to perform these processes using smaller volumes results in significant cost savings, allowing the economic operation of chemical synthesis and analysis operations. As a consequence of the reduced volume requirement, the amount of chemical waste produced during the chemical operations is correspondingly reduced.
It can be concluded that due to the reduced dimensions associated with microfluidic systems, important chemical operations can be accelerated whilst at the same instance lead to a reduction of consumption of chemicals and chemical waste.
Applications of microfluidic systems are myriad. For example U.S. Pat. No. 5,922,591 describes a miniaturized integrated nucleic acid diagnostic device and system. This device is capable of performing one or more sample acquisition and preparation operations, in combination with one or more sample analysis operations. Useful applications for microfluidic systems are in nucleic acid based diagnostics and de novo sequencing applications. International Patent Appln. WO 96/04547, published Feb. 15, 1996, describes the use of electro-kinetic operated microfluidic systems for performing electrophoretic separations, flow injection analysis and chemical reactions and synthesis steps. U.S. Pat. No. 5,942,443 discloses a range of microfluidic devices and methods for performing high-throughput synthesis and analysis, especially useful in screening a large number of different chemical compounds for their effect on a variety of chemical and biochemical systems. U.S. Pat. No. 5,858,804 provides a method of performing an immunological assay in a micro-laboratory array comprising a plurality of microchannels and chambers disposed in a solid substrate. U.S. Pat. No. 6,176,991 B1 discloses a serpentine electrophoresis channel in microchip format providing efficient, high-speed analysis of the composition of chemical samples, especially for nucleic acid sequencing.
Many methods have been described for the interfacing of fluids, e.g., samples, analytes, reagents, precursors for synthesis and buffers, towards, within or between microfluidic systems. In conventional microfluidic systems, the structures and methods used to introduce samples and other fluids into microfluidic substrates limit the capabilities of the microfluidic systems. For example, conventional microfluidic systems may include a separate sample introduction channels for introducing a sample to a microchannel for processing. The sample is first introduced into the sample channel and transported through the sample channel to the microchannel. Another method for introducing a fluid involves the use of sample wells or reservoirs in communication with the microchannel for holding a relatively larger supply of the sample. Reservoirs are structures which accommodate a significantly greater volume of fluid than the microfluidic channel. A relatively small portion of the sample supply in the sample well or reservoir is introduced into the microchannel.
The total number of samples and other fluids that can be processed on a microfluidic substrate is currently limited by the size and/or the number of reservoirs through which these fluids are introduced to the microfluidic system. A disadvantage of known structures and methods for introducing fluids into a microfluidic system is the use and transfer of a much greater volume of fluid than is needed for microfluidic analysis due to significant size inefficiencies and sample loss. Furthermore, with conventional methods of introducing fluids into microfluidic systems, it is difficult to control the amount of sample introduces that is eventually introduces into the microchannel after passing through a sample channel or a reservoir.
One method of fluidic communication with microfluidic systems is by mechanical micropumps and valves, see U.S. Pat. Nos. 6,033,191 and 5,529,465. A major disadvantage of this approach for fluidic interfacing is the complex construction and operation of these micropumps and valves. Another disadvantage is there relatively large size and internal volume when compared to the internal volume of the microchannels. Often there are multiple orders of magnitude between these two volumes and the resulting discrepancy renders micropumps unattractive to interface with a large number of small dimensional microchannels.
U.S. Pat. No. 5,173,163 describes a method and device for introducing a fluidic sample in a micro-capillary for electrophoretic separation. In this method liquid is brought into a separation capillary by introducing one end of this capillary into a vessel containing the liquid to be introduced. A combination of applied external pressures and voltages results in the transport of liquid from the vessel into the capillary. The proposed method has disadvantages. The size of the device does not allow the interfacing with a large number of microchannels, and between consecutive injections, the device needs to be cleaned, thereby considerably reducing throughput.
Another method for introducing materials in a microfluidic device is disclosed in U.S. Pat. No. 6,042,709. In this approach electrokinetic forces are employed to move a charged compound through a receiving inlet opening of the microfluidic device. A disadvantage is that the precise amount of injected liquids and substances depends upon a large number of factors which are difficult to control. One important parameter is the surface potential of the microchannel wall, which together with the applied voltage determines the liquid flow. This surface potential depends on pH of the liquid to be pumped as well as its ionic composition and even the type of ions present in the liquid. It is also a disadvantage that it does not allow the efficient interfacing with a large number of different liquids as for every injection port, a separate high voltage supply is required, together with the associated liquid channels for providing a closed electrical circuit.
A method and apparatus for performing electrophoretic experiments in a highly parallel fashion is disclosed in U.S. Pat. No. 6,103,199. Here, a plurality of separation capillaries with associated wells for receiving chemical substances in fluid form are disposed in the form of a two dimensional array. The chemical substances are dispensed from a micro titer plate into these wells by an interfacing methodology employing pressurized chambers associated with the wells to be filled. A disadvantage is that only a very small fraction of the applied liquid is actually introduced in the target microchannel, the bulk of the applied liquid drop remains behind in the well by capillary forces. As a result, most of the liquid is wasted and is not available for a consecutive chemical processing step. The effect that only a small portion of the liquid transported actually is introduced in a targeted part of a microfluidic system, can be referred to as ‘injection efficiency’, i.e. the ratio between the volume of liquid required for a particular chemical operation in a part of the microfluidic system, and the total volume of liquid required for the introductory operation.
In this particular disclosure, only sub-nanoliter amounts are required for the electrophoretic separation (i.e. the chemical operation), whilst many microliters of sample are drawn from the micro titer plate (i.e. introductory operation), yielding an injection efficiency much less than 0.001. A low injection efficiency is disadvantageous because it indicates inefficient use of chemical substances and an increased production of waste.
U.S. Pat. No. 6,090,251 provides micro-fabricated structures for facilitating fluid introduction into microfluidic devices. Fluid is introduced into a plurality of receiving wells in direct communication with associated microchannels, by the dropping of liquids into these receiving wells using pressurized gas. Besides the complexity of the required fluidic manifolds and pressurizing system, also here a disadvantage is the inherently low injection efficiency as only a very small fraction of the applied liquid is actually used in the experiment.
For the introduction of liquids in capillary electrophoresis columns implemented on chip-like devices, generally electrokinetic injection is applied. See Woolley et al., Anal. Chem. 70:684–688 February (1998), Jacobson et al., Anal. Chem. 68:720–723 March (1996), Jacobson et al., Anal. Chem. 66:2369–2373 July (1994) and Effenhauser et al., Anal. Chem. 67:2284–2287 July (1995). In this method, liquid is pumped from a first well towards the microchannel for electrophoretic separation by the application of a high driving voltage between this well and a second well located downstream. Due to the charged inner surfaces of the microchannel walls, an electroosmotic liquid flow is induced pumping liquid out of the first into the targeted microchannel. This method is referred to as ‘electrokinetic injection’ and has some specific disadvantages. One disadvantage is that if a large number of liquids need to be handled, for instance in high-throughput synthesis and screening applications, a large number of wells need to be integrated on the microfluidic device. The relatively large footprint of a typical well (about 5 mm diameter) when compared to the microchannels in which the actual chemical operation is performed (about 50 μm diameter), takes up a dominating portion of the chip surface (see U.S. Pat. Nos. 6,143,152 and 6,159,353). As the costs of microfluidic chips strongly depends on the chip surface, the required integration of wells renders this liquid injection scheme unattractive for high-throughput synthesis and screening applications.
Another disadvantage of conventional systems is that for every well, a separate electrode is required together with electronic circuitry for the application of the driving voltages. This requirement results in a complex and expensive apparatus.
Another specific disadvantage with electrokinetic injection is the fact that during the application of the driving voltage on the electrode the electrolysis of water results in the generation of hydroxyl ions (OH−), hydrogen ions (H+), hydrogen gas (H2) and oxygen gas (O2). The generated ions will affect the acidity (i.e. pH) of the liquid pumped from the first well, whilst the produced gasses potentially give rise to the formation of gas bubbles in the microfluidic system thereby destroying the experiment and eventually the microfluidic device. Besides electrolysis of the aqueous medium, any present electroactive species can degrade due to electrochemical reactions at the electrode surface. For instance, the presence of chloride ions, an ion present in most liquid media, will result in the formation of chlorine gas, which on turn can interact and potentially destroy vulnerable (biochemical) compounds present in the liquid to be injected. Also the liquid to be injected can contain electroactive constituents which can be degraded by the electrochemical processes associated with electrokinetic injection. These disadvantages can be grouped and referred to as ‘electrochemical pollution’.
Another disadvantage of electrokinetic injection is that in between consecutive experiments, the well need to be thoroughly cleaned in order to prevent cross-contamination. This required cleaning step results in a reduction of throughput and makes it difficult to implement on-line monitoring. Another disadvantage is that the liquid to be injected is subjected to a high voltage. This aspect of the absence of galvanic separation makes it virtually impossible to use electrokinetic injection for in-vivo or near-vivo applications due to the danger of electrocution. Another disadvantage is that the electrokinetic injection methods referred to, are only applicable in chip like systems produced via microfabrication technology, i.e. via the use of expensive equipment and processes also applied for the fabrication of computer chips. These methods are known to have high costs. It is desirable to provide a n interfacing methodology which is also applicable in current non chip-based capillary systems. Still another disadvantage is the low injection efficiency of electrokinetic injection. To fill a typical well, about 10–50 μl of liquid is required, whilst for a particular chemical operation only sub nanoliter amounts are required.
U.S. Pat. No. 6,130,098 discloses the movement of liquid volumes into and through microchannels by employing pressures generated by heating a volume of air in direct connection with these microchannels. A disadvantage of this fluidic interfacing method is that for a correct and efficient operation the pressure generating air chamber together with electronic heater components need to be integrated with the microfluidic system. This results in a complex device with associated large costs.
It can be concluded based on the above that current methods and systems for fluidic interfacing with microfluidic devices have particular disadvantages regarding the difficulty of integration of a large number of chemical operation nodes to interface with (i.e. electrochromatography columns, reactors etc.), relatively large required liquid volumes, low injection efficiency, electrochemical sample pollution, long rinse time between analysis steps, galvanic separation and required microfabrication technologies. Besides these disadvantages, none of the above mentioned interfacing methods allow bi-directional fluidic interfacing, i.e. transporting liquids to and from microfluidic systems. There has arisen a need in the art for providing suitable bi-directional fluidic interfacing structure that allows for the implementation of a much wider range of chemical operation steps in microfluidic systems.